$(function () {
  class BackTop {
    constructor() {
      this.init();
    }
    init() {
      this.initDom();
      this.bindEvent();
    }
    initDom() {
      this.backtop_btn = $('#backToTop');
    }
    bindEvent() {
      this.backtop_btn.on('click', () => {
        $('html,body').stop(true).animate({
          scrollTop: 0
        }, 500);
      });
    }
  }
  new BackTop();
});